Iron substituted barium titanate ferroelectric compositions and capacitors comprising same

ABSTRACT

A ferroelectric composition of matter of the formula WHERE Z may be W 6 or Mo 6 and x has a value from about 0.001 to about 0.03, having a tetragonally distorted perovskite crystal structure.

United States Patent Inventor Lothar H. Brixner West Chester, Pa.

Appl. No. 724,899

Filed Apr. 29, 1968 Patented Sept. 28, I971 Assignee E. I. du Pont de Nemours and Company Wilmington, Del.

IRON SUBSTITUTED BARIUM TITANATE FERROELECTRIC COMPOSITIONS AND CAPACITORS COMPRISING SAME 2 Claims, 1 Drawing Fig.

[1.8. CI 317/238, 3l7/258, 106/39 Int. Cl 01g 3/06, C04b 35/00 Field of Search 252/629,

[56] References Cited UNITED STATES PATENTS 3,268,783 8/1966 Saburi 106/39 X 3,426,251 2/l969 Prokopowicz 3l7/258 X Primary Examiner-Tobias E. Levow Assistant Examiner-J. Cooper Attorney-Frank R. Ortolani ABSTRACT: A ferroelectric composition of matter of the for- BACKGROUND OF THE INVENTION Compositions, known in the art, having a perovskite crystal structure, ABO are exemplified by calcium titanate, CaTiO and barium titanate, BaTiO Compounds of this class which include other compounds having an element of the +2 valence state in the A position, such as Pb, have been useful for their piezoelectric properties. To improve the piezoelectric properties, elements have been substituted into the B position of the crystal structure to form solid solutions as disclosed in US. Pat. No. 2,708,244 in which zirconium is substituted into the lead titanate crystal structure or in U.S. Pat. No. 2,849,404 in which zirconium and tin are substituted into the lead titanate crystal structure.

SUMMARY OF THE INVENTION A ferroelectric composition of matter is provided which has the formula where Z may be W or Mo and x has a value from about 0.001 to about 0.03, and a tetragonally distorted perovskite crystal structure.

BRIEF DESCRIPTION OF THE DRAWING The examples are for illustrative purposes only and are not meant to limit the invention in any manner.

The compositions are prepared by standard ceramic techniques. The ceramic discs are fitted with firmly attached electrodes of Ag, Au, Pd, or Pt and tested for their electrical properties. In this form, they are capacitors which are useful in electronic equipment. The electrodes used in the following examples are Ag.

EXAMPLE 1 After weighing out 0.0404 grams Fe O 0.0583 grams W 3.9870 grams TiO,, and 10.0000 grams BaCO these reactants are well mixed and fired at a temperature of900-1,000 C. under ambient pressure in air for 8-14 hours. The product of this firing is ball-milled under acetone, dried and pressed into pellets of about 1 inch in diameter. These pellets are subjected to a second firing at 1,000-1,400 C. in air. Each of the pellets is then fitted with firmly attached electrodes and the electrical properties of this product A, are measured. The experiment is repeated using 0.0809 grams F6 0,, 0.1 175 grams, W0 3.9267 grams T10 and 10.0000 grams BaCO This product is labeled B. The experiment is again repeated using 0.1214 grams Fe O 0.1762 grams W0 3.8660 grams TiO and 10.0000 grams BaCO This product is labeled C. A printed capacitor using the product C of this example mixed with 5 weight percent glass-forming components is prepared and measured for dielectric constant. A dense ceramic capacitor of product C is prepared and measured for dielectric constant. Barium titanate, a commercially available and widely used ferroelectric material, is tested under identical conditions for dielectric constant and a dense ceramic capacitor of pure BaTiO is also tested for dielectric constant. THe dielectric constant measure for the three products of this example, BaTiO and the three capacitors is given in table I.

TABLE I Dielectric Constant Product Formula (K) at 1 kHz.

A BaFc ,w 'l'i fl); 683 B I3aFc W Ti t) 744 C BaI-e W ,,,Ti,, 0 1303 aario, 800 Printed Capacitor olC 1 150 Ceramic Capacitor 01C 5693 Ceramic Capacitor ot' BuTit) 2049 EXAMPLE 2 The following reactants are mixed, fired and tested as described in example 1: Product A, 0.405 grams Fe 0 0.0365 grams M00 3.9875 grams Ti0 and 10.0000 grams BaC0 Product B, 0.0809 grams Fe 0 0.0729 grams M00 3.9267 grams Ti0 and 10.0000 grams BaC0 The results are shown in table 11.

The following reactants are mixed, fired and tested as in example 1: Product A, 0.0405 grams Fe 0 0.0294 grams W0 0.0182 grams M00 3.9875 grams Ti0 and 10.000 grams BaCO- Product B, 0.0809 grams Fe 0 0.0587 grams W03, 0.0365 grams M00 3.9270 grams Ti0 and 10.0000 grams BaC0 Products C, 0.1214 grams Fe 0 0.0881 grams W0 0.0547 grams M00 3.8660 grams Ti0 and 10.0000 grams BaC0 The results are shown in table 111.

TABLE III Dielectric constant (K) Product Formula at 1 kHz.

A BaFe.mWo.oozs oomzaTiosasos 5 B Bit-F60.uzWo.uos 0o.o05T1o.w0a 501 C BaFen.u3W0.ou15 0o.oo1sTio.oa0s 465 The new composition of. the general formula A is obtained by substituting Fe ions into the B position of the crystal lattice of BaTiO to replace someof the body-centered Ti ions. To retain the electroneutrality of the compound, small amounts of W or Mo are simultaneously substituted into the B position of the crystal lattice.

The resultant crystalline material is single phase, that is, none of the constituent components are in evidence in the new compound. The lattice is slightly distorted along its longitudinal axis. Typical parameter of the crystal lattice are: A,=3.998 angstroms and C,=4.030 angstroms, with a C/A ratio equal to 1.008.

.At the Curie temperature, that is, the temperature at which the ferroelectric properties of the material disappear, the crystal lattice changes from its distorted tetragonal configuration to a cubic configuration where A,=C,. The Curie temperature decreases as the amount of iron and molybdenum or tungsten in the crystal lattice is increased. The drawing is a plot of decreasing Curie temperature as the amount of iron and molybdenum or tungsten, designated as x in the formula 2 2 is increased. The shaded region below the curve is the region in which the composition assumes the distorted tetragonal configuration and exhibits ferroelectric properties. The region above the curve is the region in which the composition assumes the cubic configuration in the crystal lattice and exhibits nonferroelectric properties. A line designating room temperature is drawn on the graph because the area above the room temperature line and within the shaded area of the tetragonal configuration is particularly useful. It can be seen that the room temperature line intersects the curve of the Curie temperatures at a point corresponding to x equals 0.03. In the formula x may be as small as 0.001 and while it may be large as indicated in the drawing, compounds with smaller values are preferred. The preferred compounds are those compounds with an x value equal to 0.03 down to 0.001.

The compound of the invention exhibits excellent ferroelectric properties and is, therefore, useful as capacitors and transducers where a high dielectric constant is desired.

Since the dielectric constant of the compound of this invention tends to change only slightly as the temperature approaches the Curie temperature, the capacitors made from the composition are stable over a wide temperature range and are, therefore, very desirable in the use of electronic equipment such as radios, televisions, satellites, and missiles.

Many equivalent modifications will be apparent to those skilled in the art from a reading of the above disclosure without departing from the inventive concept.

lclaim:

1. A ferroelectric composition of matter having a tetragonally distorted perovskite crystal structure of the formula o.w o.ois 0.9s5 a 2. A capacitor comprising:

a. at least one pair of electrodes; and

b. a dielectric layer of the matter of claim 1 between said pair of electrodes.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No Dated September 28, .1971

Inventor) Lothar Brixner It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 6, "BaFc W 11 0 should read BaEe W Ti O --5 column 2, line 7,

"B8,.FC(;.O2WO T1 0 should read BaFe W Ti O.

column 2, line 8, "BaFe W Ti O" should read Bale w gli 95505 -3 column 2, line 29, "BaFc Mo 11 098 0 should read BaFe Mo Ti Q85 0 column 2, line 33, "W0 should read W0 --5 V column 2, line #0, Products C" should read Product C Signed and sealed this 11 th dav of Novembe 1972.

(SEAL) Attest:

EDWARD M.FLETCHEJR,JR. ROBERT GOTTSCHALK Attestlng Officer Commissioner of Patents 

2. A capacitor comprising: a. at least one pair of electrodes; and b. a dielectric layer of the matter of claim 1 between said pair of electrodes. 